Packet based CDMA2000 BSS can be accessed to CDMA2000 core network based on soft switch using IP (Internet Protocol) switching technologies. Compared with traditional circuit domain MSS (Mobile Switching Subsystem) system, the most prominent change of CDMA2000 core network based on soft switch lies in the separation of call control from bearer. Packet based network technology is used to replace TDM (Time Division Multiplexing) technology, traditional MSC (Mobile Switching Center) network element evolves into MSCe (Mobile Switching Center Emulator) and MGW (Media GateWay). MSCe provides the function of call control and mobility management, and MGW provides the function of media control and resources transmission and has the function of media stream operation.
MGW establishes bearer connection with SDU (Selection and Distribution Unit) in a base station system according to the acquired relevant information under the control of MSCe. This process is called bearer parameters negotiation.
Under the original TDM transmission mode, when circuit data service is being accomplished, CDMA access network needs to convert via vocoder circuit data from air interface into PCM (Pulse Coded Modulation) code stream which is then transmitted to MGW via TDM circuit. In the transmission mode of adopting IP switching technologies, CDMA access network does not need to perform conversion of coding and decoding, and directly transmits circuit data as RTP (Real-Time Transport Protocol) packet via IP network, and the structure of the protocol stack corresponding to the circuit data service is shown in FIG. 1.
During the realization of circuit data transmission, no matter the original TDM transmission mode or the current transmission mode of IP switching technologies cannot enable a user to freely switch between voice and circuit data during realizing circuit data. For example, if a user wants to realize wireless G3 fax under CDMA2000, when he/she transmits fax, operation code should be added ahead of fax number to differentiate voice service and fax service, and when he/she receives fax, a operation code for receiving fax must be set to a fixed station, so that the fixed station is set in a receiving fax mode, and after fax has been received, operation code must be set to the fixed station again so that the fixed station returns to the voice mode to answer calls. In this case, the process for a user to transmit fax is very complex and would easily have errors, which is very unhelpful to popularize wireless fax in CDMA2000 system.
Hence, in CDMA2000 network, strong demands for switching SO to circuit data during voice call exist in regions worldwide where CDMA2000 wireless fax is used.